Mid-infrared (MIR) laser spectroscopic sensors have the ability to detect and monitor trace-gas molecules. Such sensors use absorption spectroscopy to measure the concentration of gas molecules. MIR may be defined as wavelengths that fall within a range of about 3μ to about 30 μm, while near IR is generally considered to be in the range of about 0.8 μm and about 3 μm and far IR is generally considered to be in the range of 30 μm to about 300 μm. Many trace gas molecules have their fundamental rotational-vibrational absorption bands, as well as the strongest vibrational bands, occur within the MIR range, with absorption signals several orders of magnitude stronger than those apparent in the near IR range.
One problem for conventional MIR gas sensing is that they are capable of capturing only one spatial component of the field vectors (or projection). There are still properties of light that are not yet fully utilized for such sensors. Current methods focus on wavelength and amplitude.
Another key problem of MIR laser spectroscopic sensors is that they cannot fill a need for high sensitivity and high selectivity simultaneously. Each gas has a unique absorption line pattern, which allows the laser-based sensor to detect its presence. In practice, however, most gases are mixtures of different compounds, which means a series of lines will most often represent a combination of different gases within one absorption spectrum. For example, environmental monitoring often measures a combination of CO, CO2, CH4, CH2O, C2HF5, N2O and NO2, while gas pipelines usually contain a mixture of HCl, CO2, CH4, CO, NOx, CH2O. In medical uses, gases may include NO, CO, NH3, C2H6, H2S, H2O2, etc. Some of these gases have a very short lifetime and extremely low concentration in chemical reaction processes, such that detecting them with high sensitivity, accuracy, and selectivity is difficult for conventional spectroscopic techniques.
One significant limitation for trace-gas sensors is limited wavelength range. No laser can provide a sufficiently broad wavelength range to separate the absorption lines for different gases when combined together. Conventional techniques use sophisticated calibration procedures to assure satisfactory accuracy in multiple-gas detection, making such approaches prohibitively time consuming.
Other approaches to trace gas detection include electromechanical sensors, which measure an amount of current that corresponds to how much gas is oxidized at an electrode, and semiconductor detectors, which detect gas concentration from the decrease in dioxide electrical resistance. In both cases, direct exposure of the sensor to the gas is needed. Holographic gas sensors use light reflection to detect changes in a polymer film matrix that contains a hologram, and a change in gas composition can generate a colorful reflection indicating the presence of a gas molecule, but it needs illumination sources such as white light or lasers. Nuclear magnetic resonance is highly accurate, but needs a highly purified substance and a relatively long timescale. In addition, none of these technologies are able to distinguish between different gas isotopes, preventing them from being used in advanced gas exploration or biomedical applications.